lesson 6.4 reflexes...
TRANSCRIPT
Lesson 6.4
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REFLEXES AND PROPRIOCEPTION
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TOPICS COVERED IN THIS LESSON
• (a) The Reflex Arc
• (b) Proprioception and Proprioceptors
What Are Reflexes?
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Reflexes are an important part of all
physical movement. They are an
automatic, rapid, and unconscious
response to a particular stimulus.
•If the command centre, or
control, for the reflex is located in
the brain, it is called a cerebral
reflex.
•If the control is located in the
spinal cord, it is called a spinal
reflex.
Classification of Reflexes
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Reflex contraction of the skeletal muscle is not
dependent on conscious intervention by higher
centres of the brain but are a way in which the
body responds to an unexpected stimulus.
• Autonomic reflexes are mediated by the
autonomic division of the nervous system and
usually involve the activation of smooth muscle,
cardiac muscle, and glands. These reflexes
regulate such bodily functions as digestion,
elimination, blood pressure, salivation, and
sweating.
• Somatic reflexes involve stimulation of skeletal
muscles by the somatic division of the nervous
system, and include such reflexes as the stretch
reflex and the withdrawal reflex.
What Is the “Reflex Arc”?
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Neurons in our bodies transmit information
to each other through a series of neural
connections that form a pathway, or circuit.
A reflex arc is a simple neural pathway along
which an initial sensory stimulus and a
corresponding message travel.
• The stimulus from sensory neurons is sent to the
central nervous system (CNS), but there is little
or no interpretation of the signal. Few, if any,
interneurons are involved.
• The signal is transmitted to motor neurons, which
elicit a response, e.g., a knee jerk.
The Five Parts to a Reflex Arc
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• The receptor, which receives the initial stimulus
(e.g., a pinprick to the skin or a loud noise)
• The sensory (or afferent) nerve, which carries
the impulse to the spinal column or brain
• The intermediate nerve fibre (the adjustor
or interneuron), which interprets the signal and
issues an appropriate response
• The motor (or efferent) nerve, which then
carries the response message from the spinal cord
to the muscle or organ, and
• the effector organ itself (e.g., a skeletal muscle),
which carries out the response (such as removing
the hand or leg away from danger)
The Reflex Arc:
The Body’s Response to the Threat of Pain
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Proprioception and Proprioceptors
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Proprioception is a person’s ability to sense the
position, orientation, and movement of the body.
• Proprioceptors are sensory receptors found in
muscles, tendons, joints, and the inner ear that can
detect the motion or position of the entire body or
a limb by responding to stimuli from within the
organism.
• Muscle spindles and tendon organs are two
proprioceptors that continuously monitor muscle
actions and are essential components of the
neuromuscular system. They “tell” the nervous
system about the state of muscle contraction and
allow the nervous system to respond accordingly.
Muscle Spindles and the
Stretch Reflex
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Muscle spindles are sensory receptors (i.e.,
proprioceptors) within a muscle fibre that
help to maintain muscle tension and that are
sensitive to changes in muscle length (rather
than tension).
• Muscle spindles are involved in the reflex
contraction of muscles (the so-called stretch
reflex).
• The usual example of the stretch reflex action is
the knee-jerk reflex.
A Muscle Spindle
(Stretch Detector)
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The Knee-Jerk Reflex
(The Patella Reflex)
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The Crossed-Extensor Reflex:
A More Complex Reflex Action
[CATCH: FIGURE 6.17 ON PAGE 187—THE
CROSSED-EXTENSOR REFLEX]
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Golgi Tendon Organs and
the Tension Reflex
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Golgi tendon organs are found at the end of
muscle fibres that merge into the tendon itself.
GTOs detect changes in muscle tension. They
help protect the muscle from excessive tension
that might damage the muscle, joint, or both.
• A Golgi tendon organ projects to the motor
neurons located within the spinal cord. When the
change in tension is detected, an impulse is sent
along afferent neurons to the central nervous
system (CNS), where they synapse with motor
neurons of the same muscle.
• The efferent neurons instantly transmit an
impulse, causing the muscle to relax, thereby
preventing injury.
A Golgi Tendon Organ
(Tension Detector)
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